a) Field of the Invention
The present invention relates to a prepreg and a process for producing the prepreg. More particularly, the present invention relates to a prepreg using, as a base material, a particular ultra-high-molecular-weight polyethylene stretched material, as well as to a process for producing the prepreg.
b) Description of the Related Art
It is known to mold an ultra-high-molecular-weight polyethylene into a fiber, a sheet or a tape and then stretch the fiber, sheet or tape to obtain a high-strength and high-modulus polyethylene material. For example, Japanese Patent Application Laid-Open No. 130313/1984 describes a process which comprises melt-mixing an ultra-high-molecular-weight polyethylene and a wax, extruding the resulting mixture, and cooling, solidifying and then stretching the extrudate; Japanese Patent Application Laid-Open No. 101032/1985 describes a process which comprises cooling an ultra-high-molecular-weight polyethylene solution, compression-molding the resulting gel-like material, and stretching the compression-molded product; and EP 253513A1 (1988) describes a process which comprises compression-molding an ultra-high-molecular-weight polyethylene at a temperature lower than the melting point of the polyethylene and stretching the compression-molded product. The ultra-high-molecular-weight polyethylene fibers, sheets or tapes obtained by the above processes are lightweight and have a high strength and a high modulus; therefore, it is expected to use them in various applications by, for example, making them into laminates, or using them as a reinforcing material and mixing them with other materials to obtain composite materials.
The ultra-high-molecular-weight polyethylene, however, has no polar group and consequently has an inactive surface. This has made generally difficult its adhesion to other materials (the sheet- or tape-shaped polyethylene, in particular, has a restricted surface area and its adhesion to other materials has been insufficient), and the ultra-high-molecular-weight polyethylene has been difficult to make into composite materials such as prepreg and the like.